In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the change thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet. The copy sheets are collected in a stacking tray or bound or stapled together into sets of copy sheets. The bound or stapled sets of copy sheets are then stacked for presentation to the machine operator.
In commercial high speed printing machines of the foregoing type, large volumes of copy sheets are fed onto a stacking tray with the height of the stack in the stacking tray being maintained at a predetermined height by a conventional passive through beam stack height sensor. The drawback with this system is that only the high point of the stack is actually sensed and it is very costly to sense the entire stack. It is desirable to sense functional points on the stack and to do so, an active sensor system that actually touches and measures the stack height at selected spots is needed.
Conventional stack height sensors include not only non-contact optical, pneumatic or capacitive sensors, but also mechanical contact sensors. Existing optical sensors, including those utilizing reflective optics, are susceptible to contamination, variation of sensitivity due to paper type, color, curl, stack density, and location of the sheets at positions other than at the calibrated focal point of the sensor. Conventional mechanical feeler type stack sensors typically utilize a rigid finger element which is indexed into and out of engagement with the top of the stack. While these devices perform adequately, the mechanical assemblies used to index and reposition the sensing arm are relatively complex. The complexity, in turn, directly effects the overall cost and reliability of the apparatus. Also, many of the contact type stack height sensors are suitable for stacks fed from the bottom. In many application, e.g., printer disc stackers, the paper or other sheet member descends onto the stack. Such applications would effectively rule out the use of certain types of conventional contact sensors and, prior to the present invention, would have required the more costly and optical or pneumatic sensors.
Hence, there still exists the need for relatively low cost, but reliable stack height sensing apparatus that can be readily integrated with the input mechanism that feeds sheets into a stacker apparatus of a printer.
Various approaches have been devised for sensing stack heights and maintaining a predetermined copy sheet stack height. The following disclosures appear to be relevant: